I'd enjoy watching them both struggle for a bit, Beth with her wildly flailing arms and Robin trying her best to stay afloat. But considering that Robin can't swim the choice is simple. Do nothing. See if Beth would try to save Robin like she did Bianca. If she did try to save Robin then it would show me that Beth is worth saving. If she didn't it would show her true colors and I would save her, but leave her stranded on a deserted island. Either way...ROBIN DIES!